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Topic: Speedometer gearing... (Read 6023 times)

Not Camaro related - but GM nontheless...A friend has a beautiful 57 Corvette with a ZZ3 crate engine in front of a GM Saginaw (3351325 casing) 4 speed and has an inaccurate speedometer. It reads out 70mph at an actual 60pmh GPS verified speed. The speed read out discrepancy continues at all speeds apparently as well - 30mph reading 40, 50 reading 60 etc..Anyone have an idea as to how to rectify the problem?He pulled out the gear at the tranny end of the speedo cable and tells me it is white with 19 teeth.The car has a rear axle ratio of 3:70 or so according to him, and now has tires that are 205/70/15 in size..I know all kinds of gears are available to correct similar problems - is this what he needs, are they still available from GM or somewhere else readily, and if so does anyone have any idea of what part number and colour of gear he should be looking for to correct such a problem?Any assistance or advice is very much appreciated! Thanks in advance - Randy

Not Camaro related - but GM nontheless...A friend has a beautiful 57 Corvette with a ZZ3 crate engine in front of a GM Saginaw (3351325 casing) 4 speed and has an inaccurate speedometer. It reads out 70mph at an actual 60pmh GPS verified speed. The speed read out discrepancy continues at all speeds apparently as well - 30mph reading 40, 50 reading 60 etc..Anyone have an idea as to how to rectify the problem?He pulled out the gear at the tranny end of the speedo cable and tells me it is white with 19 teeth.The car has a rear axle ratio of 3:70 or so according to him, and now has tires that are 205/70/15 in size..I know all kinds of gears are available to correct similar problems - is this what he needs, are they still available from GM or somewhere else readily, and if so does anyone have any idea of what part number and colour of gear he should be looking for to correct such a problem?Any assistance or advice is very much appreciated! Thanks in advance - Randy

That sounds more like a '64-'65 Muncie (which had a 3851325 main case casting number); at any rate, with those tires (same diameter as the original 6.70-15's), a 3.70 axle takes a 22-tooth driven gear, a 3.55 takes a 21-tooth, a 3.36 takes a 20-tooth, and a 3.08 takes an 18-tooth.

John - it sounds like a GM# 3987922 gear which I now find out from elsewhere on the web should be a grey coloured unit with 22 teeth.. Hopefully my friend can try and get one up here in Northern Canada (which isn't sometimes easy) to give it a try - again your assistance is very much appreciated.You're probably also correct also about transmission number as I now hear it was out of 1965 Beaumont which probably used a Muncie and not the Saginaw variety - and he was having trouble reading the casting number off it...Randy

Most parts suppliers stock these gears. Some provide charts on which gear will get you closer to the correct speedo reading. When I went to larger tires I bought 2/3 different colors & changed them out until I got the right one. There are formulas you can use, but I found buying a few was easier...they were pretty cheap. Watch out for leaks where the speedo cable connects to the xmsn, may need to put in a new o ring after you do the swap.

Thanks Joe for the heads up on the 'O' ring - he did finally manage to order a 22 tooth gear from GM as John suggested and got it changed over and Matt as you also suspected, John was indeed correct with the 22 tooth gear fixing the problem perfectly..

Beautiful car with a very good running engine - now with an accurate speedo - what else could anyone ask for?

Thanks again to you all for the assistance - Randy & Carson (the '57 owner)!!

As an aside to this whole deal - I asked Carson (the Vette owner) how he ended up with a Beaumont 327 and transmission in his car prior to swapping in the ZZ3 engine that is in there now, and his response was "it was an original fuel injection car and I just couldn't get it running right back in the '60's so I swapped drivetrains with a guy who had a '65 Beaumont with a 327 275/4 speed...Thus the reason for the 3851325 case number that John nailed and that now is in the car.Ouch! Considering just the cost of a '57 fuelie unit these days - let alone what it would do for the value of his car now, that's gotta hurt - eh?Just shows how little stock was put on OEM drivetrains and numbers matching pieces 'back in the day'..

speaking of FI engine swaps.. .. I recall going into my local garage back in the '60's.. and seeing rows of early Corvette FI units sitting up on a high shelf. when I asked, the garage owner said they were swapped off Corvettes for carburetors at request of the owners... They were all for sale for $75 each.. with no takers.. Much later on I learned that they only required someone who understood the units function, and how to set them up to make them run *right* and get great mileage too..